ENH: simplify hashing overloads of string-types

- this revises the changes made in 95cd8ee7 to replace the
  SFINAE-type of handling of string hashes with direct definitions.

  This places a bit more burden on the developer if creating hashable
  classes derived from std::string or variants of Foam::string, but
  improves reliability when linking.

STYLE: drop template key defaulting from HashSet

- this was never used and `HashSet<>` is much less transparent
  than writing `HashSet<word>` or `wordHashSet`